The use of ophthalmic lenses for the correction of ametropia is well known. For example, multifocal lenses, such as progressive addition lenses (“PALs”), are used for the treatment of presbyopia. The progressive surface of a PAL provides far, intermediate, and near vision in a gradual, continuous progression of vertically increasing dioptric power from far to near focus, or top to bottom of the lens.
Ophthalmic lens surfaces may be designed using any of a wide variety of known methods. In one such method, a progressive surface is designed by combining two progressive surfaces. More specifically, a composite surface design is computed by adding the difference in sag value between a progressive surface and a base spherical surface for each of two surfaces to a base spherical surface. However, using this method is disadvantageous in that the resulting composite surface will not have the desired surface power profile in the cases in which the base curves of the two surfaces being combined are different. Additional optimization of the composite surface typically is required to achieve the desired add power, surface power profile, and unwanted astigmatism, or astigmatism introduced or caused by one or more of the lens' surfaces, profile. Therefore, a need exists for a lens design method that permits combining two or more surfaces and that overcomes the need for additional optimization.